My nether portal in the overworld is light up and gated to prevent mobs entering/leaving, but I recently was attacked by 10 or more zombies when I entered the nether. No skeletons, creepers or spiders.
Could it be a glitch where the pigmen are spawning as zombies? I killed 4 pigmen before the zombies appeared, and 2 were wondering the area after I slaughtered the hoard.
A big wave of pigmen with a single zombie attacked. Then a zombie villager appeared behind me in my small pigman hunting lodge which contains the portal. A couple more zombies appeared outside. This happened twice but the second time there were more zombies and less pigmen in the initial wave. I also had another wave without a zombie in my bunker.
I think it might be an event like the zombie hoards that attack villages.
Zombies have been dropping from higher platforms that can't be accessed from the portal platform and I have stayed close to the bunker so I doubt they have been coming through it. Especially because there has not been a single skeleton, creeper or spider.
Update. I just saw a zombie spawn beside a pigman in a lit area. I had not hit any pigmen yet.
Snocrach also said:
Zombie pigmen can summon their zombie friends from the overworld to help them fight the forces of evil.
this is a normal minecraft feature - nothing to do with the world generation.
I am playing the Snocrash's Hermit Chunk Challenge in 1.9